The main reasons you will need to think about print orientation is for aesthetics, strength, and supports.
- With FDM printing you will get the greatest amount of detail in the Z direction. (The vertical direction) The reason that is, is due to the how a printer is constructed. The standard nozzle diameter is 0.4 mm but in the Z you can have layer heights of 0.1 mm or smaller. That is a gain of 75% or more detail. The down side to this is it usually takes longer to print and is overall weaker.
- Strength is the next thing to keep in mind. When you are trying to decide the best orientation based on strength, it is best to think of a 3d print like a piece wood. The layer lines in a 3d print act like the grain structures in wood. The layers of a 3d print will all ways be the weakest part of your model. You wouldn't build a bridge with vertical end grain so you shouldn't do the same with a 3d print. Ideally you would want your grain to be perpendicular to the load. This isn't always possible in a signal print so if strength is something you're after, it might be best to break up your model with that in mind.
- In an ideal world you should always aim for zero support structures to cut down on printing time and post processing but that is usually easier said than done. Some tips for designing files that don't need supports is to avoid overhangs (Features that angle out more that 60 degrees or features thatl start printing in midair before joining your part) and use bridging to your advantage. Sometimes supports are impossible to avoid but if you rotate your model sometimes you can limit the amount of support your will need to generate and remove.